In recent years, R&D efforts have been intense on both LED chip and phosphors for phosphor-converted LED (pcLED), with the result that both efficient high-power LEDs and efficient phosphors have been demonstrated. However, a unique aspect of the phosphors operating in pcLED is that the phosphors are in close vicinity of the LED chip, and the LEDs operate at high temperatures. Typical junction temperatures of high power LEDs are in the range of 100° C.-150° C. At these temperatures, the crystal of the phosphor is at a high vibrationally excited state, causing the excitation energy to be directed to heat emission through lattice relaxation rather than to the desired luminescence emission. Moreover, these lattice relaxations produce heating with vibrational excitation, and thereby further reduce the luminescence emission efficiency. This is a vicious cycle that precludes successful applications of existing phosphor materials. The pcLED lamp for general illumination application requires high optical energy flux (e.g., higher than 1 Watt/mm2) which causes additional heating by a Stokes shift generated inside the phosphor crystals. Successful development of pcLED lamps for general illumination, therefore, requires phosphors that can operate highly efficiently at temperatures of 100° C.-150° C. The risk is that it is difficult both to achieve 90% quantum yield at room temperature and to have high thermal stability at 100° C.-150° C. The thermal stability of a phosphor's luminescence is an intrinsic property of the phosphor which is determined by the composition and the structure of the crystalline material.
Oxynitride phosphors have been considered for use in pcLEDs because of their excellent luminescence performance at high temperature range mentioned above. Prominent examples are the sialon-based phosphors whose host crystals are constituted by chemical bonds of Si—N, Si—O, Al—N and Al—O as the backbone of the host crystal structure. Each of the oxynitride phosphors discovered so far comprises predominantly a single crystalline phase, and often a second phase is considered an “impurity”. However, phosphors are generally materials that permit non-stoichiometric proportions and are usually heterogeneous. In this invention, a group of oxycarbidonitride phosphor compositions are demonstrated to be comprised of more than one unique crystalline phase, each of which fluoresces highly efficiently.
The introduction of carbon or carbide into crystalline phosphor materials has previously been considered detrimental in luminescence performance. The often dark body color of various carbides may be a source of absorption or quenching of emission light. Also, residual unreacted carbide that remains after phosphor preparation utilizing carbon or carbide processes can hinder the emission intensity of the phosphor.
Carbidonitride phosphors are comprised of carbon, nitrogen, silicon, aluminum and/or other metals in the host crystal and one or more metal dopants as a luminescent activator. This class of phosphors recently emerged as a color converter capable of converting near UV (nUV) or blue light to green, yellow, orange and red light. The host crystal of carbidonitride phosphors is comprised of —N—Si—C— networks in which the strong covalent bonds of Si—C and Si—N serve as the main structural components. Generically, the network structure formed by Si—C bonds has a strong absorption in the entire visible light spectral region, and therefore has been previously considered not suitable for use in host materials for high efficiency phosphors. For example, in certain nitride-silicon-carbide phosphors in which Ce3+ is the dopant, the electronic interaction between Ce3+ and the —N—Si—C— networks results in a strong absorption in 400-500 nm wavelengths, making the phosphor less reflective in that particular spectral region of visible light. This effect is detrimental to achieving a phosphor having a high emission efficiency.
It has now been discovered that in certain oxycarbidonitride phosphor compositions, carbide actually enhances, rather than quenches, the luminescence of a phosphor, in particular at relatively high temperatures (e.g. 200° C.-400° C.). The invention demonstrates that the reflectance of certain oxycarbidonitride phosphors in the wavelength range of visible light decreases as the amount of carbide increases. These carbide-containing phosphors have excellent thermal stability of emission and high emission efficiency.